1. FIELD OF THE INVENTION
The present invention relates to a flow rate measuring device, in particular a device for measuring the mass flow rate of intake air into an internal combustion engine.
2. DISCUSSION OF BACKGROUND
Before explaining prior art, a typical problem in the induction system for automobile engines will be explained. In FIG. 36 is shown a schematic cross-sectional view of the structure of an induction system for a typical automobile engine. In this FIG., reference numeral 1 designates an intake air flow rate measuring device for measuring the flow rate of intake air, and the reference numeral 31 designates a sensor in the device, or a flow rate detecting element. Reference numeral 2 designates an air cleaner, reference numeral 3 designates an air filter, reference numeral 4 designates arrows to indicate the flow direction of the intake air from the atmosphere, reference numeral 5 designates dirt and dust mingling in the air as contaminants, reference numeral 6 designates a bellmouth for increasing trapping efficiency, reference numeral 7 designates a throttle valve for controlling an intake air quantity in cooperation with an accel pedal (not shown), reference numeral 8 designates a surge tank, reference numeral 11 designates an engine combustion chamber, reference numeral 20 designates an induction pipe for introduction of the intake air, reference numeral 9a designates an intake manifold for introducing the intake air into the engine combustion chamber 11, reference numeral 9b designates an exhaust manifold for exhausting exhaust gas after combustion, reference numeral 10a designates an intake valve, reference numeral 10b designates an exhaust valve, and reference numeral 12 designates wiring to be used for the flow rate measuring device 1.
In the induction system, the intake air is filtered by the entire surface of the air filter 3 as shown as air streams in the air cleaner 2 indicated by arrows 13a, 13b, 13c, 14a, 14b and 14c. Since the intake air flow rate measuring device 1 is normally provided in the vicinity of the outlet of the air cleaner, the flow rate measuring device 1 can be clearly affected by a change in the air streams in the air cleaner 2 due to clogging of the air filter 3.
In FIG. 37 is shown a schematic view to explain how air streams are created when the dirt and dust 5 has deposited on the air filter 3. When the dirt and dust 5 is taken into the induction system as shown in FIG. 36, the dirt and dust 5 has deposited in the vicinity of the inlet of the air cleaner 2 in a large amount to clog the air cleaner, significantly changing the air passage. The air streams in the air cleaner 2 in such a case are indicated by arrows 15a, 15b, 15c, 16a, 16b and 16c. The clogged portion of the air filter 3 on the inlet side can not permit the air to pass, and the intake air passes through unclogged portions of the air filter 3.
When the dirt and dust 5 has deposited on the air filter 3 as stated above, vortexes are generated as shown by the arrows 15a and 16a, and the portions of the air filter 3 through which the intake air passes changes significantly. In such a case, irregular vortexes mingle with the air flow into the flow rate measuring device 1 to increase a disturbing degree, changing the flow velocity distribution or the non-uniform flow distribution generated in the initial state. In addition, swirling streams increase to provide more complicated streams for the following reasons.
The flow rate value that is outputted from the flow rate measuring device 1 in the initial state with neither dirt nor dust 5 deposited on the air filter 3 is a value detected measured by the flow rate detecting element 31, and the detected value is treated as a represented value of the flow rate in the induction pipe 20, which is regarded as the quantity of the air actually taken into the engine. The air cleaner 2, which is actually mounted onto various kinds of cars, is usually formed in an asymmetric three-dimensional unique shape as one example shown as a plan view and a side view in FIGS. 38(a) and 38(b) Since the air cleaner 2 has an inlet 2a and an outlet 2b formed at asymmetric positions thereon, the air entering the flow rate measuring device 1 not only has a deviation in the velocity distribution but also contains many swirling streams having a central axis in the flow direction. If the air filter is clogged, the asymmetry of the flow passage has become enhanced to increase the revolving component in the swirling streams.
To sum up, if the air filter 3 is clogged, the disturbing degree increases to enhance the noise in output values, the distribution in the non-uniform flow distribution changes, and the revolving component in the swirling streams alter to change the centrifugal force by the rotation, modifying the flow velocity distribution. As a result, the flow rate that is detected at the flow rate detecting element 31 changes, creating a problem to produce an error in flow rate detection.
Since the dirt and dust accumulated on the air filter 3 gradually increases according to the length of the running time of the engine in general, the introduction system changes with the lapse of time. When the air cleaner 2 that has changed with the lapse of time is used, the flow rate measuring device 1 produces an error in detection to fail optimized control of an air-fuel ratio. As a result, the purifying function in an exhaust emission control system, such as a three-way catalyst, is adversely affected to introduce serious air pollution. The three-way catalyst is one that can provide the best purifying efficiency with respect to nitrogen oxides NOx, hydrocarbon HC and carbon monoxide CO in such a state that the air-fuel ratio is controlled in optimized fashion. The emission regulation has been determined to be severer throughout the world for the twenty-first century, and an issue of a change in the introduction system with the lapse of time is predicted to become a big issue more and more.
In addition, a change in the introduction system with the lapse of time, variations in the assemblage of the air filter 3 or the air cleaner 2 change the flow velocity distribution in the air flow to create a problem in that an error in flow rate detection is produced. It is the task of the flow rate measuring device 1 to detect a flow rate correctly irrespectively of the structure of a flow passage or the structure of an induction system.